Koohoolinn Koohoolinn - 4 months ago 59x
Java Question

How to handle a Findbugs "Non-transient non-serializable instance field in serializable class"?

Consider the class below. If I run Findbugs against it it will give me an error ("Non-transient non-serializable instance field in serializable class") on line 5 but not on line 7.

1 public class TestClass implements Serializable {
3 private static final long serialVersionUID = 1905162041950251407L;
5 private Set<Integer> mySet; // Findbugs error
7 private HashSet<Integer> myOtherSet;
9 }

That's correct because java.util.Set never implements Serializable in its hierarchy and java.util.HashSet does.
However it is best practice to code against interfaces instead of concrete implementations.

How can I best handle this?

I can add a @Suppresswarnings(justification="No bug", values="SE_BAD_FIELD") on line 3. I have quite a lot of Sets and Lists in my actual code and I'm afraid it will litter my code too much.

Are there better ways?


However it is best practice to code against interfaces instead of concrete implementations.

I submit that no, in this case it is not. Findbugs quite correctly tells you that you risk running into a NotSerializableException as soon as you have a non-serializable Set implementation in that field. This is something you should deal with. How, that depends on the design of your classes.

  • If those collections are initialized within the class and never set from outside, then I see absolutely nothing wrong with declaring the concrete type for the field, since fields are implementation details anyway. Do use the interface type in the public interface.
  • If the collection are passed into the class via a public interface, you have to ensure that they are in fact Serializable. To do that, create an interface SerializableSet extends Set, Serializable and use it for your field. Then, either:
    • Use SerializableSet in the public interface and provide implementation classes that implement it.
    • Check collections passed to the class via instanceof Serializable and if they're not, copy them into something that is.